Question

I been working in a project for my Data Networks course and I encounter a memory leak but I don’t understand why it’s happening.

By the way, I know there is a awful mix of C and C++, but I can’t do anything about it, it’s based on class code and I can’t modified that, I know that this is not a good way to do it and I need to use char* as requisite.

My program it’s multithreaded and I handle this struct:

typedef struct packetQueue
{
    char* buf;
    int length;

    packetQueue()
    {
        buf = nullptr;
        length = 0;
    }

    packetQueue(char* buffer, int len)
    {
        length = len;
        buf = new char[length + 1];
        memcpy(buf, buffer, len);
        buf[length] = '\0';
    }

    packetQueue(const packetQueue& other)
    {
        length = other.length;

        if (other.buf)
        {
            buf = new char[length + 1];
            memcpy(buf, other.buf, length);
            buf[length] = '\0';
        }
        else
        {
            buf = nullptr;
        }
    }

    packetQueue& operator=(const packetQueue& that)
    {
        if (this == &that)
        {
            return *this;
        }

        delete[] buf;

        length = that.length;

        if (that.buf)
        {
            buf = new char[length + 1];
            memcpy(buf, that.buf, length);
            buf[length] = '\0';
        }
        else
        {
            buf = nullptr;
        }
        return *this;
    }

    ~packetQueue()
    {
        delete[] buf;
        buf = nullptr;
    }

} PACKET;

In my constructor with two parameters I do that assignment because the push of my queue does a deep copy of my struct and like I have my copy constructor and I already handled that. So, I have one thread (I been testing one by one, and the VLD results are just for this one).

DWORD _stdcall PHY_in_Thread(void* data)
{
    int numbytes, counter = 0;

    SOCKET hostSocket = *(SOCKET*) data; // Socket where the host receives

    struct sockaddr_in si_recvfrom;
    struct sockaddr_storage their_addr;
    socklen_t addr_len;

    addr_len = sizeof their_addr;

    while ( 1 )
{
    char* recBuf = new char[BUFLEN + 1];
    // Checks if it's any buffer on the socket to be processed
    if ( (numbytes = recvfrom(hostSocket, recBuf, BUFLEN, 0, (sockaddr*) &si_recvfrom, &addr_len)) == -1)
    {
        cerr << "Could not receive datagram." << endl;
        delete[] recBuf;
        closesocket(hostSocket);            
        WSACleanup();
        exit(0);
    }
    recBuf[numbytes] = '\0'; // append NULL to the end of the string

    char* temporalBuffer = new char[numbytes - CHECKSUM_MAX_SIZE + 1];
    memcpy(temporalBuffer, recBuf, numbytes - CHECKSUM_MAX_SIZE);
    temporalBuffer[numbytes - CHECKSUM_MAX_SIZE] = '\0';

    char extractedChecksum[CHECKSUM_HEX_SIZE + 1];
    DWORD crcBuffer = crc32buf(temporalBuffer, numbytes- CHECKSUM_MAX_SIZE); // Calculates the CRC32 checksum
    _snprintf(extractedChecksum, 8 , "%08lX", crcBuffer); // Prints the string in a buffer
    extractedChecksum[CHECKSUM_HEX_SIZE] = '\0';

    delete[] temporalBuffer;

    string strExtractedChecksum = extractedChecksum; // Copies the array in a string
    transform(strExtractedChecksum.begin(), strExtractedChecksum.end(), strExtractedChecksum.begin(), upper); // Uppercase the string

    // Array for store the checksum of the packet
    char readChecksum[CHECKSUM_MAX_SIZE + 1];

    // Store the checksum of the packet in local variable
    memcpy( readChecksum, &recBuf[numbytes - CHECKSUM_MAX_SIZE], CHECKSUM_MAX_SIZE);    
    readChecksum[CHECKSUM_MAX_SIZE] = '\0';

    std::stringstream stream;
    string strReadChecksum;
    for (int i = 0; i < CHECKSUM_MAX_SIZE; i++ )
    {
        int number = static_cast<int>(readChecksum[i]); // Casts every character of the checksum array

        if ( readChecksum[i] <= -1 ) // In case the int value it's negative adds the constant value to make that recognizable
        {
            number += 256;
        }

        // Convert the decimal number in a hex representation
        stream.str("");
        stream << hex << number;

        if ( stream.str().length() < 2 ) // In case it's a number less than 10, adds a 0 at the beginning
        {
            strReadChecksum += "0" +  stream.str();
        }
        else
        {
            // Working out the presentation of the number
            strReadChecksum += stream.str();
        }
    }

    std::transform(strReadChecksum.begin(), strReadChecksum.end(), strReadChecksum.begin(), upper); // Uppercase the string
    strReadChecksum[CHECKSUM_HEX_SIZE] = '\0';

    cout << "[PI] Frame #" << counter <<" received ("<< numbytes <<" bytes). " << endl;
    if ( !strcmp(strReadChecksum.c_str(), extractedChecksum) ) // Checks if the CRC are equal
    {
        cout << "[CRC] Checksum OK: 0x" << extractedChecksum << endl;
    }
    else
    {
        cout << "[CRC] Checksum failure: 0x" << extractedChecksum << endl;
    }

    // Push the packet in the MAC_in_queue to be processed
    MAC_in_queue.push(PACKET(recBuf, numbytes));
    recBuf = nullptr;
    counter++;

            break;   // Just for test one packet
}

MAC_in_queue.clear();

return 0;

}

But when I execute this thread and send something to this thread to be stored in this queue gives a leak. In this execution there is only one item to make things simple.

---------- Block 29 at 0x0068F718: 264 bytes ----------
  Call Stack:
    d:\program files (x86)\microsoft visual studio 11.0\vc\include\concurrent_queue.h (402): Host.exe!Concurrency::concurrent_queue<packetQueue,std::allocator<packetQueue> >::_Allocate_page + 0xF bytes
    f:\dd\vctools\crt_bld\self_x86\crt\src\concurrent_queue.cpp (113): MSVCP110D.dll!Concurrency::details::_Micro_queue::_Push + 0xD bytes
    f:\dd\vctools\crt_bld\self_x86\crt\src\concurrent_queue.cpp (240): MSVCP110D.dll!Concurrency::details::_Concurrent_queue_base_v4::_Internal_move_push
    d:\program files (x86)\microsoft visual studio 11.0\vc\include\concurrent_queue.h (581): Host.exe!Concurrency::concurrent_queue<packetQueue,std::allocator<packetQueue> >::push + 0xF bytes
    d:\users\silex rpr\documents\visual studio 2012\projects\project3\hoster\host.cpp (638): Host.exe!PHY_in_Thread + 0x3D bytes
    0x7474339A (File and line number not available): kernel32.dll!BaseThreadInitThunk + 0x12 bytes
    0x76EC9EF2 (File and line number not available): ntdll.dll!RtlInitializeExceptionChain + 0x63 bytes
    0x76EC9EC5 (File and line number not available): ntdll.dll!RtlInitializeExceptionChain + 0x36 bytes
  Data:
    00 00 00 00    01 00 00 00    60 F8 68 00    80 00 00 00     ........ `.h.....
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........
    CD CD CD CD    CD CD CD CD    CD CD CD CD    CD CD CD CD     ........ ........

But I don’t understand where it’s this data leaking, I hope I made myself clear.

Thanks beforehand

Answer 1

At the beginning of while ( 1 ) loop in PHY_in_Thread() you allocate recBuf:

char* recBuf = new char[BUFLEN + 1];

but at the end of the loop body, you miss to free it; you just leak the memory due to
recBuf = nullptr; assignment.

Instead, try with proper recBuf deletion at the end of loop body:

delete [] recBuf;
recBuf = nullptr;